A conventional exhaust gas purification apparatus is formed by disposing, in an exhaust passage of an internal combustion engine, a selective reduction type catalyst (an SCR catalyst) and a reducing agent adding valve for adding a reducing agent constituted by an ammonia (NH3) precursor (an aqueous solution of urea, ammonium carbamate, or the like) to exhaust gas.
In a proposed technique for detecting deterioration of an exhaust gas purification apparatus such as that described above, deterioration of the selective reduction type catalyst is determined on the basis of a NOx purification ratio of the selective reduction type catalyst when the internal combustion engine is in a steady state operating condition and a time required for transient variation in the NOx purification ratio to stabilize in a transient condition (see Patent Document 1, for example).
Patent Document 2 describes a technique of specifying an amount of NH3 actually adsorbed to the selective reduction type catalyst in a high temperature region where an NH3 adsorption capacity of the selective reduction type catalyst decreases, and determining that the selective reduction type catalyst has deteriorated when the specified amount of NH3 is equal to or smaller than a threshold.
Patent Document 3 describes a technique of keeping an addition amount per predetermined time period constant by increasing an addition frequency while shortening an opening interval of a urea water adding valve.
Patent Document 4 describes a technique of modifying an atomized particle size of a urea water solution when the temperature of the selective reduction type catalyst is in a predetermined low temperature region by increasing an injection pressure at which the urea water solution is injected by a reducing agent adding valve.
Patent Document 5 describes a technique of achieving atomization of a reducing agent by supplying the reducing agent from a reducing agent adding valve when a peak of an exhaust gas pressure wave reaches a position of the reducing agent adding valve.